This invention relates to communication systems and, more specifically, to modulation techniques for communication systems.
Typical communication systems transmit information from one location or source to a second location or destination. The information travels from the source to the destination through a channel; this channel is typically a noisy channel. Thus, the channel introduces various forms of noise. The term “noise” is used herein to define various forms of signal corruption, such as interference, fading, attenuation, environmental impact, and electronic noise, that alter the characteristics of a signal as it travels through a channel. Accordingly, the signal that is transmitted through the channel and received at a receiver is a combination of the transmitted signal and the effects of noise introduced by the channel as a result of travelling through the channel.
In a cellular communications system, one type of noise is called “interference”. More specifically, there are at least two forms of interference in communication systems: co-channel interference (CCI) and inter-symbol interference (ISI). CCI arises in communication systems due in part to the fact that there are several transmitters in communication with the same receiving unit. The signal from one transmitter can interfere with the signal from another transmitter. For example, in a cellular communication system there are several mobile stations in communication with the same base station which often leads to CCI. Each transmitter is an omni-directional transmitter. However, a signal being transmitted from one transmitter can take several paths as the signal travels from the transmitter to the receiver. This leads to ISI, a form of self interference.
As indicated above, in a communication system information is transmitted through the channel from the source to the destination. The information is carried by a carrier signal that is modulated to contain or carry the information. Various forms of modulation are used for transmission of the information through the channel. Modulation is the process of varying the characteristic of a carrier according to an established standard or scheme; the carrier is prepared or “modulated” by the information to produce a “modulated” carrier signal that is transmitted by the source to the destination through the channel. For example, in a cellular communication system, modulation is the process of varying the characteristics of the electrical carrier as information is being transmitted. The most common types of modulation are Frequency Modulation (FM), Amplitude Modulation (AM), and Phase Modulation (PM).
One modulation technique currently used in the industry is called Orthogonal Frequency Division Multiplexing (OFDM). OFDM is one of the techniques for multicarrier modulation. Multicarrier modulation is a technique for modulating multiple carriers with different information, all of which are transmitted simultaneously or parallel in time. OFDM has high spectral efficiency as well as tolerance to multipath fading. As indicated above, transmitters are omni-directional and transmit in all directions. Thus, a signal emerging from a transmitter, or the source, can travel multiple paths to reach the receiver, or the destination. Accordingly, multipath fading occurs on a carrier signal's intensity, which results in alteration of the information being carried.
Typically, the information bearing signal itself is referred to as the baseband signal, when it is transmitted without a carrier. Sometimes the baseband signal has to be embedded in a high frequency carrier and communicated. Then, the high frequency carrier signal that delivers the information bearing (baseband) signal through suitable modulation is usually referred to as the passband signal.
The efficiency of a system utilizing OFDM stems from the simultaneous or parallel transmission of several subcarriers in time. While this lowers the bit-rate on each of the subcarriers, it provides an “N”-fold increase in aggregate bit-rate, wherein “N” is the number of subcarriers. Additionally, because the low bit-rate signals hardly suffer any ISI and the subcarriers are orthogonal, it is possible to demodulate the subcarriers independent of each other. A conventional OFDM system comprises a set of sub-symbols X[k] transmitted in time using an Inverse Fast Fourier Transform (IFFT). The time-domain baseband signal can be represented as:
            x      ⁡              [        n        ]              =                  1                  N                    ⁢                        ∑                      k            =            0                                N            -            1                          ⁢                              X            ⁡                          [              k              ]                                ·                      exp            ⁡                          (                                                j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  k                  ⁢                                                                          ⁢                  n                                N                            )                                            ,      n    =    0    ,            1      ⁢                          ⁢      …      ⁢                          ⁢      N        -    1  
Thus, the N-sample long transmitted OFDM symbol vector can be expressed as:xN=IFFT{XN}where, xN and XN are the time and frequency domain symbol vectors, respectively.
In a typical OFDM system, binary symbols or bit streams are encoded in the form of complex valued numbers. The complex valued numbers are drawn from an M-ary alphabet. The complex valued numbers are then used to modulate a set of orthogonal sub-carriers to generate a time-domain signal using an Inverse Discrete Fourier Transform (IDFT). The resulting baseband signal, which is usually complex valued, is quadrature modulated on a Radio Frequency (RF) carrier and transmitted through an air interface channel. The transmitted signal is corrupted by channel noise and dispersion before being received.
There are several problems associated with systems that utilize OFDM modulation techniques. For example, the channel is subject to fading due to multipath and path loss. Additionally, the channel suffers from ISI which poses a problem at the receiver when data has to be detected. Furthermore, manufacturers of devices that transmit and receive data are always faced with the challenge of increasing the amount of and the rate at which information can be transmitted over a finite bandwidth while overcoming signal loss due to channel noise.
One of the persistent problems with OFDM systems is a high peak to average power ratio (PAR or PAPR). The PAR is a measure of the peak power that occurs in the time domain OFDM signal relative to the average power transmitted. A high PAR is usually difficult to handle and involves undesirable power-throughput tradeoffs due to imperfect RF power amplifiers (RFPA). The RFPA's have to operate in output back-off modes, leading to lower output power and reduced throughput or capacity. What is more, operating an RFPA in back-off modes leads to very low power efficiencies, and excessive heating in transmitters.
The most common method to reduce system PAPR is by clipping the signal whenever the envelope amplitude exceeds the clipping threshold. The problem with this technique is two fold. First, the signal fidelity is lowered because of signal energy that has been discarded by clipping the peak-valued signal samples. Second, any clipping action is an amplitude compression scheme leading to a bandwidth expansion (however subtle) in the frequency domain. Furthermore, the effectiveness of clipping decreases as we employ higher orders of modulation such as 16-QAM or 64-QAM.
Companding is another method that yields impressive PAPR alleviation. The bandwidth expansion associated with these methods is however significantly higher than clipping. Also, such methods do not lend themselves very well to implementation in multipath channels. Coding methods have also been tried in the past to reduce PAPR. The tradeoff is however again in a reduced effective code-rate of the system, which is again a price paid in bandwidth.
Therefore, there is a great incentive to reduce the OFDM PAR as it can lead to system wide throughput & power efficiency gains. What is needed is a system and method for minimizing signal PAR, the impact of ISI and fading on OFDM systems, as well as enhancing the bit-rate or spectral efficiency.